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DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THK 



Bureau of Standards 

S. W. STRATTON. Director 



No. 203 

INFLUENCE OF PHOSPHORUS UPON THE MICRO- 
STRUCTURE AND HARDNESS OF LOW- 
CARBON, OPEN-HEARTH STEELS 



BY 



EDWARD C. GROESBECK, Associate Chemist 
Bureau of Simdards 



NOVEMBER 21, 1921 




PRICE, 10 CENTS 

SoW otay by the Superintendent of Documents. Government Printine OfBce 
Washington, D, C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1921 



^Vi. •. ,-. 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 203 

INFLUENCE OF PHOSPHORUS UPON THE MICRO- 
STRUCTURE AND HARDNESS OF LOW- 
CARBON, OPEN-HEARTH STEELS 



BY 



EDWARD C. GROESBECK, Associate Chemist 

Bureau of Standards 



NOVEMBER 21, 1921 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents. Government Printing Office 
Washington. D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1921 



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-7 






INFLUENCE OF PHOSPHORUS UPON THE MICRO- 
STRUCTURE AND HARDNESS OF LOW -CARBON. 
OPEN -HEARTH STEELS 

By Edward C. Groesbeck 



ABSTRACT 

In this investigation, undertaken with the object of throwing additional light en 
the role played by phosphorus upon the properties of low-carbon, open-hearth steels 
which are often used in the manufacture of products where severe cold working is 
employed during fabrication, no clear relationship could be established between 
the phosphorus content, varj-ing within the range 0.008 to 0.115 percent, which marks 
the usual limits in plain carbon steels, and the microstructure and hardness as devel- 
oped in two series of specimens, one of basic open-hearth steel and the other of acid 
open-hearth steel, by a series of different heat treatments, because of the marked 
irregularity in the distribution and grain size of the ferrite and pearlite grains found 
present in many of the specimens. This irregularit>^ was traced, by means of the 
microstructure as developed by etching, to the nonuniform distribution of the phos- 
phorus. A cellularlike structure formed in conjunction with the microstructure 
normal to these steels was studied and relationship between this unusual structure 
and the distribution of phosphorus was established. 

Because of the very limited amount of material available for this investigation it 
was not possible to extend the scope of the work beyond the study of the micro- 
structure and hardness of the steels in question. 



CONTENTS 

Page 

I. Introduction 2 

II. Aim of investigation 3 

III. Preparation of material 3 

1 . Material 3 

2 . Apparatus 4 

3 . Experimental procedure 4 

IV. Microstructure 7 

1. General structure 7 

2. Irregularity of structure 12 

3. Relation between grain size and phosphorus content 12 

4. Effect of rate of cooling upon the pearlitic structure 17 

V. Hardness 17 

1. Experimental data 17 

2. Analysis of results 17 

(a) Shore 17 

(6) Brinell 19 

VI. Distribution of phosphorus 19 

1. Steels with lower phosphorus content. . . . ; 19 

2. Steels with higher phosphorus content 25 

3. Interpretation of results 32 

VII. Summarj^ and conclusions 3s 

I 



2 Technologic Papers of the Bureau of Standards [Voi. i6 

I. INTRODUCTION 

In connection with an investigation made several years ago 
into the effect of varying phosphorus content upon the endurance 
quaHties of low-carbon steel when severely cold worked, as in 
stamping, drawing, pressing, upsetting, and bending. Dr. J. S. 
Unger ^ prepared two series of ingots, one of basic open-hearth 
steel and the other of acid open-hearth steel, all having the same 
general composition, but with the phosphorus content varying from 
0.008 to 0.1 1 per cent, and which comprises the range of phos- 
phorus content usually found in acid open-hearth and acid Besse- 
mer steel. These steel ingots were rolled down into the usual 
billets, slabs, and sheet bar preparatory to the formation by cold 
work of the various products upon which the tests were made. 
These steels were given no other than the customary heat treat- 
ment accorded ordinary soft steels during the process of fabrication. 
Through Dr. Unger's kindness, a small quantity of material in the 
form of I -inch rounds from the two series of ingots was made 
available for this study. 

There appears to be scant information of a systematic nature 
in the published literature with regard to the influence of varying 
amounts of phosphorus, especially within the limits usually found 
for this element in open-hearth and Bessemer steel, upon the 
microstructure and physical properties of low-carbon steel as 
developed by heat treatment. D'Amico published in 191 3 ^ the 
results of an investigation carried out on the microstructure, hard- 
ness, mechanical, and magnetic properties as developed by heat 
treatment in a series of 1 2 low-carbon steels, made by the electric 
process, to which phosphorus had been added, so that the resulting 
phosphorus content varied from 0.012 to 1.242 per cent. Only 
two of his steels come within the range of phosphorus content 
covered in this investigation. Dr. Unger ^ stated that in his 
investigation the effect of heat treatment was not studied, inas- 
much as soft steels are rarely heat treated, though he made a few 
tensile and other tests on some heat-treated bars. 

1 J. S. Unger, Proceedings Amer. Iron and Steel Inst., 1918, pp. 172-193; also Iron Age, 101, p. 1538; June 
13, 1918. 

^ E. D'Amico, " Uber den einfluss des phosphors auf die eigenschaften des flusseisens," Ferruni, 10. pp. 
2S9-J04. 

'' See footnote i. 



Groesbeck] 



Phosphorus in Open-Hearth Steels 



II. AIM OF INVESTIGATION 

The scope of this investigation, undertaken with the hope of 
throwing additional Hght on the role played by phosphorus upon 
the properties of the steels in question or, at least, of the ordinary , 
low-carbon steels which are used for the manufacture of products 
where severe cold working is employed during fabrication, was 
confined to the relationship between the phosphorus content, as 
present within the limits 0.008 to 0.115 per cent, and the micro- 
structure and hardness as developed by a series of heat treatments 
and also the distribution of the phosphorus in the steel as revealed 
by metallographic etching. 

III. PREPARATION OF MATERIAL 
1. MATERIAL 

The steels used in this investigation were of the following general 
chemical composition : Carbon, 0.12 per cent; manganese, 0.36 per 
cent; silicon, 0.020 to 0.022 per cent; sulphur, 0.036 to 0.037 
per cent; copper, 0.012 to 0.014 P^^ cent; and phosphorus as 
below : 



Basic open-hearth series 

Per cent 

Steel A o. 008 

Steel B 030 

Steel C 052 

Steel D 080 

Steel E .' no 



Acid open-hearth series 



Per cent 



Steel BB o. 032 

steel CC 058 

Steel DD 085 

Steel EE 115 



The limited amount of material made available for this study 
necessitated the use of small samples. Transverse slices one-fourth 
inch thick were cut from the i -inch rounds and these slices in turn 
radially into quarters. Each specimen thus presented a face 
approximately a quadrant of a circle of one-half inch radius for 
microscopic examination and hardness determination. 

The nine specimens, one from each of the steels in. both the 
B. O. H. and A. O. H. series for each heating, were fastened three 
in a row to a piece of rather coarse iron wire gauze, which was 
wrapped around the pyrometer tube at the closed end in such a 
way that the three rovv^s of specimens were placed approximately 
equidistant around the outer circumference of and in contact with 
the pyrometer tube and also so that the hot junction of the chromel- 
alumel thermocouple, used in conjunction with a millivoltmeter, 
was appro.ximately centrally located within the cluster of the nine 
specimens. 



4 Technologic Papers of the Bureau of Standards [Voi. lO 

2. APPARATUS 

All but one of the heatings were carried out in a horizontal 
tubular nichrome-wound electric furnace, provided with an iron 
pipe 2 3/^ inches inside diameter and 27 inches long, over which 
was slipped a fairly snug fitting alundum cylinder 24 inches long 
and around which the heating element was wound. The equaliza- 
tion of the temperature within the hotter part of the iron-pipe 
furnace tube was aided by means of baffle plates, and the speci- 
mens to be heated were placed within the portion of the furnace 
tube found by trial to have the flattest thermal gradient, namely, 
about 5° C over a distance of 2^ inches. Boats filled with ground 
charcoal were placed within the furnace tube near both ends in 
order to counteract the decarburizing conditions as far as prac- 
ticable. In the lead-bath heating, which was carried out in a gas- 
fired, lead-bath furnace, the specimens were placed around the 
nichrome pyrometer protection tube at the closed end in a similar 
manner as in the other heatings, and this tube was placed in such 
a position that the specimens were approximately in the center 
of the molten lead bath, which was about 6j/< inches in diameter 
and 1 1 inches deep. 

3. EXPERIMENTAL PROCEDURE 

A series of six heat treatments was made, in which the rate of 
cooling from the maximum temperature, 900° C, was so regulated 
that the cooling through the 750 to 600° C range occupied the 
following periods of time : Four hom-s, 2 hours, i hour, 30 minutes, 
15 minutes, and 5 minutes. In all cases the cooling from 900 to 
750° C was so controlled that the rate of cooling in this range 
would correspond as closely as practicable to that followed through 
the 750 to 600° C range, which was chosen as that in which the 
formation and the break-up of the lamellar pearlite into "granular" 
pearlite occurred in order to note whatever influence the phos- 
phorus content might have on this tendency. In the lead-bath 
heating (HT-7) the gas and air were shut off entirely soon after 
the maximum temperature, 900° C, was reached, and the cooling 
of the bath and specimens was allowed to proceed uninterruptedly 
in the furnace until the temperature of the bath had reached 500° C 
when the specimens were taken out and allowed to cool in the air. 

Fig. I shows graphically the heating and cooling curves for all 
the seven heatings carried out. 



Groesbeck] 



Phosphorus in Open-Hearth Steels 



HT-[ : 4 HOURS 




\. 



I I 1 1 1 ^ 1 1 1 1 1 1 1 — 

40 80 IZO 160 ZO0 Z40 Zao 3ZO J60 400 -44o 480 5ZO 



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HT-2: 2 HOURS 



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red h«r furnacft 



HT-5: I HOUR 



^ 



■-od hot furnace 



O tZO >60 Zoo 240 Z80 3Z0 40 SO IZO 160 ZOO 




TIME = Minutes 

Fig. I. — Heating and cooling diagrams for the series of heat treatments carried out 

See Table i for the lengths ol time covered by the various stages of the heatings and coolings. The 
'times given in the title for each of the diagrams refer to that taken in cooling down through the 750-600° C 
lange. 



6 Technologic Papers of the Bureau of Standards [Voi i6 

In Table i are given the periods of time occupied by the different 
stages of the heatings and coolings. 

TABLE 1.— Heat Treatment— Times of Heating and Cooling 



Laboratory number 


Desired 
rate of 
cooling 


Condition of furnace on 


Heating interval 


Cooling interval 




through '"=■<=. i.u.i ui spcuiiiicub 

750-600" C 


To 900° C 


At 900° C 


900-750° C 


750-600° C 


HT-1 


Minutes 

240 

120 

60 

30 

IS 

5 

MS 


Cold 


Minutes 

145 
61 
14 
137 
110 
35 
42 


Minutes 
12 
15 

12 
12 
15 
14± 
clOi 


Minutes 
241 
109 

61 

35 

nvs 

2± 
16± 


Minutes 
234 


HT-2 


Red hot 


116 


HT-3 


do 


63H 
29A 


HT-4 


Cold 


HT-5 


Hot 


14H 

15 


HT-6 


Red hot " 


HT-7 


Red hot 









a The pipe container with the specimens was put back into the furnace immediately following a heating 
to 900° C and cooling down below 500° C, as the cooling through the 750 to 600° C range was found to have 
been too fast. 

6 Lead bath. 

c During the holding at 900° C, the air blast stopped for a moment, so the temperature fell from 900° C, 
where it had been for about four minutes, down to 862° C, and it was raised again to 900° C, remaining at or 
above that temperature for six minutes more. 

In the HT-5, 15 minutes, and HT-6, 5 minutes, heat treatments, 
an iron pipe 1^4 inches in diameter and 10 inches long overall, 
with iron caps screwed on at both ends, was used as a container 
for the specimens. Iron rods were fastened lengthwise to the con- 
tainer at the bottom in order to give a clearance between the 
furnace-tube wall at the bottom and the caps at both ends of the 
container and to facilitate the passage of air underneath in the 
1 5 -minute cooling. The fused silica pyrometer tube was inserted 
through a hole in the cap at one end of the container. The 
container was so situated in the furnace tube that the cluster of 
specimens placed in the center of the container would lie within 
the zone where the thermal gradient was flattest. 

In the HT-5, 15 minutes run, the cooling of the pipe container 
and specimens was effected by means of a stream of air (laboratory 
table air blast) introduced through silica tubes placed through 
both ends of the furnace tube, one directed at the upper edge of 
one end and the other at the lower edge of the opposite end of the 
container. 

In the HT-6, 5 minutes run, the pipe container was at the start 
of the cooling shoved out of the furnace tube into a 12 -inch long 
and 3 -inch diameter alundum tube surrounded by pipe insulation 
covering that had been preheated to a couple of hundred degrees 



Groesbeck] PhospkoTus in Open-Hcarth Steels 7 

centigrade above room temperature and both open ends of the 
alundum tube were then immediately plugged up in order to check 
the outflow of the heat. In this manner the cooling of the speci- 
mens through the 750 to 600° C range was so regulated as to take 
about five minutes. 

IV. MICROSTRUCTURE 

In the grinding of the heat-treated specimens for microscopic 
examination and hardness determination ample allowance was 
made for the decarburization produced during heat treatment, 
and the extent of which was ascertained by measuring the thickness 
of the decarburized layer at one edge of a copper-plated and rather 
heavily ground specimen that had been given the longest heating 
(HT-i) carried out in the series of heatings. 

1. GENERAL STRUCTURE 

It was noted from a study of micrographs, taken of all the 
heat-treated specimens at near each corner of the quadrant- 
shaped specimen, that there is a marked irregularity in the dis- 
tribution and size of both the ferrite grains and pearlite kernels 
in many of the specimens, especially in the A. O. H. steels as 
compared with that for the B. O. H. steels, and that there is no 
definite relation between the occurrence of this irregularity of 
structure and the phosphorus content and heat treatment. 

Figs. 2, 3, 4, and 5, which were selected from the steels having 
the minimum and maximum phosphorus content (the 0.008 per 
cent phosphorus B. O. H. steel possessed a similar structure and 
grain size to that of the 0.030 per cent phosphorus B. O. H. steel 
for all the seven heatings) and were cooled through the 750 to 
600° C range in 4 hours, 30 minutes, 5 minutes, and in the lead 
bath, illustrate in a general way the structure as developed in the 
different steels by the various heat treatments tried. For the 
steels of intermediate phosphorus content and cooled at the 
other rates tried, the structure is similar to that shown for the 
higher or lower phosphorus content and the more slowly or 
quickly cooled steels. There were several cases in which there 
was a marked deviation from the general structure as outlined 
above, and these cases are indicated in Table 2, where there is a. 
large increase in the average grain area, as compared with that 
for the more slowly or quickly cooled specimens for the same steel. 

It should be borne in mind that the grain size, as shown in 
Figs. 2 to 5, represents the effect, or rather the grain growth, 

G3.S78°— 21 2 



Technologic Papers of the Bureau of Standards \Voi.i6 




'^~k^ 



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i^. 



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D, Ic^ad bath 



Fig. 2. — Micro siructiirc of low-carbon basic open-hearth steel containing 0.030 per cent 

phosphorus. Xioo 

Heated to 900° C and cooled through 750-600° C range in the periods of time as indicated. Etching 
reagent, s per cent alcoholic solution of picric acid. 



Groesbcck] 



Phosphorus in Open-Hearth Steels 



"W^ 






4^ 



A. 4 hours 






.^ 



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i 



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D. lead bath 



Fig. 3. — Micro structure of low-carbon acid open-hearih steel containing 0.0J2 per cent 

phosphorus. Xioo 

Heated to 900° C and cooled through 750-600° C range in the periods of time as indicated. Etching 
reagent, 5 per cent alcoholic solution of picric acid. 



lo Technologic Papers of the Bureau of Standards Voi.i6 



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Fig. 4. — Microslrnciure of low-carbon basic open-hear/h siwl containing o.iio per cent 

phoxpJwriis. y.ioo 

Heated to 900° C and cooled through 750-600° C range in the periods of time as indicated. Etching 
reagent, 5 per cent alcoholic solution of picric acid. 



Groesbcck] 



Phosphorus in Open Hearth Steels 



II 




4 hours 



■f%? 



\.^ 






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•- ^.' 






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^.^'v. -'»• 







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A J 



Fig. 5. — Microsiructure of low-carbon acid open-heaith steel containing o.llj per cent 

phosphorus. Xioo 

Heated to 900° C and cooled through 750-600° C range in the periods of time as indicated. Etching 
reagent, 5 per cent alcoholic solution of picric acid. 



12 



Technologic Papers of the Bureau of Standards [Voi. i6 



produced during the interval of time above the A3 point 
occupied by the specimens in the course of the heat treatment. 
The data in the table given below are quite in agreement with 
what might be expected as to the relative grain size developed 
in the various specimens during the different periods of heating 
and cooling above the A3 point (assuming Acg for the two series 
of steels used in this investigation to be at 860° C) with the 
exception of the HT-7, lead -bath specimens, which show a 
marked increase in grain size even to a greater extent in all cases 
but one than in the case of the most slowly cooled series of speci- 
mens (HT-i, 4 hours). No satisfactory explanation can be 
offered as to the reason why the much shorter time of interval 
(33 minutes) above 860° C should develop an appreciably larger 
grain size in the HT-7, lead -bath specimens, than that developed 
in the specimens which were more slowly heated and cooled in 
the electric furnace (HT-i, 4 hours, and HT-4, 30 minutes). 



Heat treatment (see Table 1) 



Time of 

interval 

above 

860° C 



Average grain size in m' (Table 2) 



B. O. H. per cent P 



0.030- 0.110 



A. O. H. per cent P 



0.032 0.115 



HT-1, 4 hours 

HT-4, 30 minutes 

HT-6, 5 minutes 

HT-7, 15 minutes (lead bath) 



Minutes 
96 
50 
26 
33 



911 

767 

589 

1736 



1075 

1019 

714 

2111 



3304 
1728 
1329 
1682 



1161 
924 
990 

1713 



2. IRREGULARITY OF STRUCTURE 

In several of the specimens there were found present in the 
structure large areas (amounting in extent to about 15 per cent 
of the area of the microscopic field) which were quite free from 
pearlite and had small to very large ferrite grains, as Figs. 6 and 7 
will show. These carbonless areas appear to be due to the pres- 
ence of a greater concentration of phosphorus, arising from non- 
uniformity in the distribution of the phosphorus in the steel which 
will be discussed in a later section of this article. 

3. RELATION BETWEEN GRAIN SIZE AND PHOSPHORUS CONTENT 

In Table 2 are given the results of the grain-size determination 
made of all the heat-treated specimens. The planimetric method 
of grain-size determination as modified and described by Jeffries* 



*Z. Jeffries, Trans. Am. Inst. Min, Engrs., 54, p. 594; 1916. Also Met. and Chem. Eng., 18, p. 185; 1918. 



Groesbeck] PhospJioYus in Open-Hearth Steels 1 3 




Fig. 6. — Typical examples of irregularity in microstructnre as found present in many of 

the specimens. Y.IGO 

All the specimens were heated to 900° C and cooled through the 750-600° C range in the periods of time 
as indicated below. Etching reagent, 5 per cent alcoholic solution of picric acid. A— Low-carbon basic 
open-hearth steel containing 0.052 per cent phosphorus— 30 minutes; B— Low-carbon basic open-hearth 
steel containing o.iio per cent phosphorus— i hour; C— Low-carbon acid open-hearth steel containing 
0.1 15 per cent phosphorus — 2 hours. 



14 



Technologic Papers of the Bureau of Standards [Vd. lo 



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» ' *-. ■• ; > 











■N^-^- 



1 

t -• 



Z hours 






■»,■ 






...^.», 






^ 



i^- 








2 hours 



Fig. 7. — Microstructure of low-carbon acid open-hearth steel containing 0.032 percent 

phosphorus. Xioo 

Heated to 900° C and cooled through 750-600° range in the periods of time as indicated. C is of the same 
specimen as A, but taken at a different spot. Shows carbonless areas as found in a few of the specimens, 
though particularly in specimens from this steel. Etching reagent, 5 per cent alcoholic solution of picric 
acid. 



Groesbeck] Pkospkorus tfi Open-Hearth Steels 15 

was followed. The pearlite kernels were counted together with 
the ferrije grains in determining the average area of the grains, 
the pearlite kernel being assumed as a grain. In comparing the 
average size of the pearlite kernels with that of the ferrite grains, 
the pearlite kernels appear, as estimated by the eye, to be about 
one-quarter in size of that of the ferrite grains for the quickly 
cooled sets of specimens (15 minutes and 5 minutes — B. O. H. 
and A. O. H.), one-half in size for some of the more slowly cooled 
sets (4 hours, 2 hours, i hour, 30 minutes, lead bath — B. O. H. 
and 30 minutes, lead bath — A. O. H.) and one-half to full size 
in the remaining slowly cooled sets (four hours, two hours, one 
hour^A. O. H.). The ratio of the number of ferrite grains to 
pearlite kernels (F:P) has also been determined, and the results are 
given in Table 2. There appears to be a well-defined and marked 
decrease in this ratio in the relatively quickly cooled specimens 
(15 minutes, 5 minutes, and lead bath) as compared with that 
prevailing for the more slowly cooled samples, and this is quite in 
accordance with what might be expected when one considers the 
opportunities the carbon has for being precipitated out of solid 
solution or austenite and the resulting pearlite masses to coalesce 
into larger masses during the cooling from 900° C, or above A3, 
down through the transformation range. No relationship is 
seen to exist, however, between the ratio of ferrite grains to 
pearlite kernels (F:P) and the phosphorus content. 
63378°— 21 3 



i6 



Technologic Papers of the Bureau of Standards [Voi. 16 






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Groesbeck] PhosphoYus ill Open-Hcarth Steels 17 

Because of the irregularity in the grain-size results, which was 
evidently due to the nonuniform distribution of the phosphorus in 
the steels, no definite conclusions could be drawn as to the rela- 
tionship between the grain size and phosphorus content and also 
the effect of various rates of heating and cooling followed in the 
series of heat treatments. 

4. EFFECT OF RATE OF COOLING UPON THE PEARLITIC STRUCTURE 

With respect to the effect of the rate of cooling through the 750 
to 600° C range, the divorcing of pear lite had progressed to some 
extent in the more slowly cooled specimens (4 hours, 2 hours, i 
hour, and 30 minutes, especially in the steels with higher phos- 
phorus content) and this divorcing and the coalescing of the lib- 
erated cementite appear to have been more pronounced in the 
B. O. H. steel specimens than in the A. O. H. ones. However, 
there appears to be no well-defined relation between the degree of 
divorcing developed and the phosphorus content, within the limits 
0.008 to 0.1 15 per cent phosphorus. 

The undivorced pearlite was found to be mainly in the sorbitic 
and sublamellar ^ conditions for both B. O. H. and A. O. H. series 
of specimens, though in the more slowly cooled specimens (four 
hours, two hours, and one hour) there was a considerable quan- 
tity of fine lamellar pearlite present. 

V. HARDNESS 
1. EXPERIMENTAL DATA 

Both the Shore and Brinell hardness values were determined, 
and the results are given in Table 3 . In the Brinell test the speci- 
mens were too small and thin to permit the use of a 3000 kg 
load without distortion arising from the indentation of the speci- 
men, so a 500 kg load was used instead with entire satisfaction. 

2. ANALYSIS OF RESULTS 

(a) Shore. — ^The results, which are rather irregular, show no 
well-defined relation between the hardness and the phosphorus 
content and also the rate of cooling through the 750 to 600° C 
range. The range between the minimum and maximum indi- 
vidual values was 12.0 to 21.0 = 9.0, and 70 per cent of the indi- 
vidual values lay between the limits 17.0 and 19.0. 



' H. M. Howe and A. G. Levy, "Notes on pearlite," Jour. Iron and Steel Inst., 94, p. 220; 1916, II. 



i8 



Technologic Papers of the Bureau of Standards [Voi. ic 



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Groesbeck] Phosphovus 171 Open-Hearth Steels 19 

(&) Brinell. — A more satisfactory relationship was obtained 
between the hardness and phosphorus content. In all of the 
seven differently heated sets of specimens there is a small but 
definite increase in hardness, as the phosphorus content is raised 
from 0.008 to 0.080 per cent in the B. O. H. series, though the 
hardness remains practically stationary as the phosphorus content 
is increased to o.iio per cent, excepting in the one hour and five 
minute sets, where the increase in hardness continues. In the 
A. O. H. series the results are rather erratic, though in a general 
way the hardness increases a little as the phosphorus content is 
increased from 0.032 to 0.058 per cent and then remains prac- 
tically stationary as the phosphorus is increased to 0.115 per cent. 
In the four-hom- set the increase in hardness is practically con- 
tinuous as the phosphorus content is increased from 0.032 to 
0.115 per cent. In most of the sets of heat-treated specimens, 
both B. O. H. and A. O. H., the increase in hardness is quite in 
agreement with the rate of increase as indicated in the statement 
by Stead " that the Brinell hardness of saturated solid solution of 
iron phosphide in iron increases with the phosphorus content by 
about 1.3 points for each o.oi per cent phosphorus. D'Amico^ 
reports an increase of hardness by about 12 Brinell hardness 
numbers for each o.i per cent phosphorus. 

No well-defined relationship could be established between the 
hardness and the rate of cooling through the 750 to 600° C range, 
though the more slowly cooled specimens (four hours, two hours, 
and one hour, B. O. H. and A. O. H.) were, in general, a little 
softer than the more quickly cooled ones. 

VI. DISTRIBUTION OF PHOSPHORUS 
1. STEELS WITH LOWER PHOSPHORUS CONTENT 

As stated in Section IV (2), there were met with, during the 
microscopic examination of the entire series of specimens pre- 
pared for this investigation, a number of specimens in which 
large carbonless or free from pearlite areas and abnormally 
large ferrite grains were found present in portions of the micro- 
section. These irregularities in structure were attributed to a 
nonuniform distribution of the phosphorus, there being a greater 
concentration of this element present in these carbonless and 

« stead, " Some of the ternary alloys of iron, carbon, and phosphorus," Jour. Soc. Chem. Ind., 33, p. 174; 
1914. 
^ See foot note 2. 



20 Technologic Papers of the Bureau of Standards \Vui. i6 

large ferrite-grains areas than in the surrounding metal. In 
addition, in several of these large carbonless areas there was 
noticed an unusual structure distributed throughout the cluster 
of ferrite grains and which appeared to stand out in relief as 
viewed through the microscope using vertical illumination.^ This 
structure may be seen in Figs. 8, a, and 9, d, and it was clearly- 
seen when the rest of the microscopic field was in focus. On closer 
examination this relief structure was found to form a part of a net- 
,work and mesh system of markings, very suggestive of a cellular 
structure, distributed over the rest of the microsection, though the 
throwing a little out of focus of the microscopic field was neces- 
sitated in order to bring out clearly this system of markings, as 
Fig. 8, b, c, and Fig. 9, a, b, c will illustrate. The network stood out 
as light-colored markings and the meshes appeared as of a darker 
color, while a reversal of the focusing slightly beyond the point 
of sharp focus and in a direction toward the specimen gave the 
opposite color effect, though the contrast between the light and 
dark portions was less pronounced." The cell walls of this net- 
work appear to cross the grain boundaries of the ferrite grains in 
places, thus indicating the presence of two distinct systems of 
etching patterns, the usual ferrite and pear lite system and the 
cellularlike system, the latter apparently being independent of 
and superposed on the former. This cellularlike structure was not 
developed in the case of the 0.008 and 0.030 per cent phosphorus 
B. O. H. steels even after fairly prolonged etching (20 to 25 minutes 
with 5 per cent alcoholic picric acid and 30 seconds with 2 per cent 
alcoholic nitric acid), though it was readily brought out in the 

' The author's attention was called, subsequent to the preparation of this paper, to G. F. Comstock's 
article " Microstructure of annealed soft steels, ^vith special reference to phosphorus in tin plate, " published 
in Forging and Heat Treatment, 7, pp. 60-63, Jan., 1921, in which he reports the presence of faint relief 
markings in the ferrite areas comprising the "ghost streaks" in specimens of steel sheet bar containing 
o.ii per cent carbon and 0.075 per cent phosphorus that had been annealed for four hours at Soo and 850° C, 
or within the Aci to An range, though this unusual structure was not shown as being present in specimens 
of the same steel annealed at 900° C. and at higher temperatures. On etching with Stead's reagent portions 
of this relief structure were darkened while the rest of the ferrite remained bright, thus indicating that 
phosphorus was responsible for this peculiar structure. Although it is true that the specimens used in this 
investigation were heated to 900° C, it would seem from the fact that the cellularlike structure persisted in 
many of the specimens examined that the time of heating to and above Acs was not long enough to produce 
a complete diffusion of the phosphorus and consequent elimination of the relief or cellularlike structure. 
No attempt was made, however, to investigate this point fully, though the cellularlike structure was found 
to be more pronounced in the rapidly cooled specimens than in the slowly cooled ones. 

' This color contrast as occasioned by the unequal depth of etching produced is in agreement with F. 
Osmond's observation, as stated onp.iyof his Microscopic Analysis of Metals, edited by J. E. Stead and 
published by Charles Griffin & Co. (Ltd.), London, 1904, that cementite particles assume a dark color as 
contradistinguished with the light-colored ferrite background when the focusing of the microscope is carried 
toward the specimen and not quite to the point of focus. The network of the above-described cellularlike 
structure represents the portion of the metal which is lower in phosphorus content, as will be shown in a later 
section, and hence has been eaten away to a greater extent by the etching medium than the adjoining 
higher-phosphorus portions which stand out in relief. 



Grocsbcck] 



Phosphorus in Open-Hearth Steels 



21 







&C-. 






A. 



X (00 



B. 



X 300 




i 




C. 



X 300 



l^iQ 8.—Cellularlike structure in low-carbon acid open-hearth steel containing 0.032 per 

cent phosphorus 

Heated to 900° C and cooled through 750-600° C in 15 minutes. B was taken near center of field shown 
in A, and field is in sharp focus. C is same field as in B, but thrown a little out of focus to bring out 
cellularlike structure. Etching reagent, 5 per cent alcoholic solution of picric acid. 



22 



Technologic Papers of the Bureau of Standards 



[Vol. i6 




X 100 



Fig. 9. — Cellularlike structure in same specimen shown in Fig. 8 hut at a different portion 

of niicrosection 

B was taken at center of field shown in A, and the field is in sharp locus. C is the same field as in B, 
but thrown a little out of focus to bring out cellularlike structure. D was taken at another spot in same 
microsection. Etching reagent, s per cent alcoholic solution of picric acid; D was also etched with 
Stead's reagent following the picric acid etching. 



Groesbeck] PkosphoTus ill Opsn-Heavth Steels 23 

0.032 per cent phosphorus A. O. H. steel and in the other steels, 
both B. O. H. and A. O. H., containing the higher percentages of 
phosphorus. This fact suggests that the cellularlike structure 
will not be developed unless the phosphorus content of the steel 
be greater than about o.oi or 0.02 per cent. 

In order to prove that the carbonless areas, large ferrite grains, 
and cellularlike structure were due to the phosphorus, etching 
with alcoholic cupric chloride solution acidified with hydro- 
chloric acid (Stead's reagent), which was found by trial to give 
more satisfactory results than several of the usual reagents recom- 
mended for the detection of phosphorus segregation such as 
Heyn's reagent, Rosenhain and Haughton's reagent, Stead's 
aqueous solution of picric acid, and heat tinting, was tried follow- 
ing the etching with picric acid or nitric acid. It is well known 
that through the action of Stead's reagent the portions of the 
etched surface low in phosphorus content have a layer of metallic 
copper or a brown-colored layer deposited thereon while the por- 
tions richer in phosphorus remain colorless or nearly so because 
of the greater resistance to the action of the etching."- "> ^- As to 
the nature of the brown-colored deposit, Rawdon '' states that 
this coloration or tint, attributed bv Stead to a much-retarded 
deposition of copper, as represented by the darkened surface 
layer when dissolved off from the etched specimen gave a faint 
yet clear test for copper. In no case where etching with Stead's 
reagent was used in this work was there any metallic deposit of 
copper formed on the specimens. 

Fig. 9, d, and Fig. 10, a, h, c, are typical of the results obtained by 
etching with Stead's reagent. It will be noted that the large 
carbonless areas and large individual ferrite grains remain prac- 
tically colorless while the surrounding areas assume a mottled 
appearance, which on closer examination and at higher magnifi- 
cation was seen to possess a similar though less pronounced etch 
pattern as that developed in the 0.42 per cent phosphorus steel 
specimens described in the next subsection. However, in the 
case of the lower phosphorus steels (0.008 and 0.030 per cent 
phosphorus B. O. H.) the etch pattern was not very pronounced 
or definite, evidently because of the insufficient amount of phos- 
phorus present. 

'" stead, "Some of the ternary alloys of iron, carbon, and phosphorus," Jour. Soc. Chem. Ind., 33, p. 174, 
1914; seventh section of Part I; also Jour. Iron and Steel Inst., 91, p. 174, 1915, I. 

11 Rawdon, " Some unusual features in the microstructure of -vvrouccht iron," Trans. Amer. Inst. Mining 
Engrs., 588, p. 501, 1918; also B. S. Technologic Paper No. 97. 

1- Whiteley, " The distribution of phosphorus in steel between the points Aci and Acs," Jour. Iron and 
Steel Inst., 101, pp. 363 and 370-372; 1920, I. 

1' See footnote 11. 



24 



Technologic Papers of the Bureau of Standards [Voi. i6 




A. 



X 50 



B. 



K (00 











C. X 100 



Fig. 10. — Carbonless areas and large ferriie grains in low-carbon acid open-hearth steel, 
as brought out by etching with Stead's reagent 

The specimen represented by A and B contained 0.032 per cent phosphorus, and was heated to 900° C 
and cooled through 750-600° C range in 2 hours. The specimen represented by C contained 0.058 per cent 
phosphorus, and was heated to 900° C and cooled through 7-0-600° range in 5 minutes. B was taken in 
the same microsection as A but at a different spot. Etching reagent, 5 per cent alcoholic solution of 
picric acid followed by Stead's reagent. 



Groesbcck] Phospkofus in Opefi-H earth Steels 25 

2. STEELS WITH HIGHER PHOSPHORUS CONTENT 

For the purpose of determining whether or not a similar and 
more pronounced cellularlike structure as developed by etching 
with either picric or nitric acid and etching pattern as formed by 
Stead's reagent would be obtained in steels of higher phosphorus 
content, a series of specimens cut from a three-fourths-inch round 
of the following composition: 0.34 per cent carbon, 0.42 per cent 
phosphorus, o.io per cent silicon, o.ii per cent manganese, and 
0.029 per cent sulphur that had in connection with another inves- 
tigation been heated to both 900 and 800° C and cooled through 
the 800 to 650° C range at various rates (two hours, one hour, 
accelerated furnace cooled, and air cooled) were studied. 

A cellularlike structure similar to that found in the lower phos- 
phorus steels was found present in the two-hour and one-hour 
cooled specimens of both the 900 and 800° C heatings (see Fig. 
II and Fig. 12, a, b) and in the accelerated furnace-cooled and 
air-cooled specimens of the 800° C heating, though in these last 
two specimens this cellularlike structure was quite clearly seen 
only at higher magnification because of the much finer granular 
structure (ferrite and pearlite) developed by the quicker rates of 
cooling. It should be noted that this cellularlike structure is 
absent in different spots where there is a cluster of more or less 
large pearlite kernels, and this is apparently due to a lower phos- 
phorus content occasioned by the repellant action which carbon 
dissolved in iron is known to exert upon phosphorus. This as- 
sumption appears to be borne out in Fig. 13, h, and Fig. 15, c, 
where the areas surrounding the pearlite kernels have been deeply 
tinted by Stead's reagent, thus indicating a lower phosphorus 
content in these regions. 

As for the results obtained by etching with Stead's reagent, 
Fig. 13, a, c show the general manner of distribution of the phos- 
phorus as found in the two-hour and one-hour cooled specimens, 
the uncolored or light portions being richer in phosphorus than 
the dark portions. Figs. 12, c, and 13, 6 show in greater detail 
the arrangement of the low (dark) and high phosphorus (light) 
areas. The phosphorus-rich areas shown in Fig. 13, a, c appear 
to be the originally landlocked portions that had formed between 
the dendrites of purer iron or primary crystallites during the 
solidification of the metal, though the dendritic structure is not 
very marked evidently because of the working given the metal 
during its fabrication into three-fourths-inch rounds. It appears 



26 



Technologic Papers of the Bureau of Standards [Voi.i6 




Fig. II. — Cellularlike sirucinre in medium carbon steel contain- 
ing 0.42 per cent phosphorus, y.100 

Heated to 900° C and cooled through 800-650° C range in i hour. B is 
same field as shown in A, but thrown alittle out of focus. Etching reagent, 
5 per cent alcoholic solution of picric acid. 



Grocshrcl;] 



Phosphorus in Open-Hearth Steels 



27 




Fig. 12. — Cellularlike striicticre in medium carbon steel containing o./f2 per cent phosphorus, 
before and after etching itith Stead's reagent. X500 

Same specimen as in Fig. 11, but taken at different portion of microsection. A, B, and C are of the' 
same field, but B has been thrown a little out of focus. Etching reagent, s per cent alcoholic solution 
of picric acid; C was etched with Stead's reagent following the picric acid etching. 



28 



Technologic Papers of the Bureau of Standards \Voi.i6 




A 



X 75 



B 



X 250 




C. 



X 30 



Fig. 13. — Distribution of phosphorus {light areas) in medium carbon steel containing 

o.-P per cent phosphorus, as revealed by etching with Stead's reagent 

The specimen represented by A and B was heated to 900° C and cooled through the 800-650° C range in 
I hour (same specimen as in Figs, ir and 12), and that by C was heated to 900° C and cooled through 
800-650° C range in 2 hours. B was taken at center of field shown in A. Etching reagents: A and B, 
5 per cent alcoholic solution ol picric acid followed by Stead's reagent; C, 2 per cent alcoholic solution of 
nitric acid followed by Stead's reagent. 



Groesbcck} PkosphoTus tu Open-Hearth Steels ■ 29 

probable that after the solidification had been completed the 
carbon diffused into the purer portions or the dendritic spines, 
leaving the other impurities as phosphorus and sulphur in the less 
pure portions or the landlocked areas as may be well illustrated 
in Fig. 7, c, where the carbon in the form of pearlite lies outside 
of a good-sized zone free from pearlite and containing numerous 
inclusions of manganese sulphide (cross-sectional view of the 
I -inch round). 

In the accelerated furnace-cooled and air-cooled specimens of 
the 900° C heating the phosphorus was found to be distributed in 
a different manner, as indicated in Fig. 14, a, and the phosphorus- 
rich portions (phosphoferrite or a solution of iron phosphide in 
iron) to be in the form of small acicular and globular-shaped 
masses grouped together in clusters, as shown in Figs. 14, b, c and 
15, c. Fig. 15, a, b cover the same field as that in Fig. 15, c, 
but show the microsection as etched with nitric acid alone, thus 
revealing a light-colored etching pattern which corresponds 
exactly with the dark or low phosphorus portions of the etch 
pattern in Fig. 15, c. 

It will also be noticed that the light-colored cell walls of the 
cellularlike structure in Fig. 12, b correspond exactly in position 
with and overlap the dark-colored etch-pattern in Fig. 12, c, 
though the latter is, in general, of greater width along the various 
cell walls. The light-colored network and dark-colored etch 
pattern are seen to cross the ferrite grain junctions in many places, 
and a similar tendency may be found in Fig. 13, b, thus confirming 
the statement made above (Sec. VI, i) that the cellularlike 
structure appears to be independent of the ferrite and pearlite 
structure. 

Fig. 14, c shows the structure of the 900° C air-cooled specimen, 
which had been heat tinted to a good blue color. The light- 
colored, phosphorus-rich portions stand out in good relief against 
the dark oxidized surface of the surrounding and poorer in phos- 
phorus metal, and appear to be quite similar in general formation 
to that prevailing in the 900° C accelerated furnace-cooled speci- 
men (Fig. 14, h). It may be of interest to note here Whiteley's 
statement" that areas containing as much as 0.6 per cent phos- 
phorus are readily detected by heat tinting. 

" Loc. cit. (see footnote 12), p. 375. 



30 



Technologic Papers of the Bureau of Standards [Voi. i6 




Fig. 14. — Disfrihutinn of phosphorus in medium carbon, steel coniainivq 0.43 per cent 

phosphorus 

The specimen represented by A and B was heated to 900° C and coded ciuickly in the turnacc, and that 
by C was heated to 900° C and air-cooled. Etching reagents: A, 2 per cent alcoholic solution of nitric 
acid; B, Stead's reagent; C, heat-tinted in molten lead bath at 300° C to good blue color. 



Grocsbcch] 



Phosphorus in Open-Hearth Steels 



31 




Fig. 15. — Phosphorus-rich area shou-n in Fig. 14 (B). before and after etching leith 

Stead's reagent. X500 

A, B, and C are of the same field, which was taken at center of the cluster of uncolored spots shown in 
lower half of Fig. 14 (B). though B has been thro-^\-n a little out of focus. Etching reagent, 2 per cent 
alcoholic solution of nitric acid; C was etched with Stead's reagent following the nitric-acid etching. 



32 Technologic Papers of the Bureau of Standards [Voi.id 

3. INTERPRETATION OF RESULTS 

It must be evident from the fact that the etch pattern as de- 
veloped by Stead's reagent corresponds exactly in position with 
and overlaps the cellularlike structure as developed by picric or 
nitric acid etching that both systems of etch patterns are due to 
one and the same cause, the presence of phosphorus. This element 
is found to be more concentrated in the meshes of both etch 
patterns, these meshes appearing as dark areas in the cellularlike 
structure and as light areas in the etch pattern produced by Stead's 
reagent. This conclusion appears to be further borne out by the 
following considerations : 

(a) Both the cellularlike structure and Stead's reagent etch 
pattern are more pronounced in the higher phosphorus steels than 
in the lower phosphorus steels. 

{b) The large uncolored areas or meshes of the Stead's reagent 
etch pattern tend to be further away than nearer to the pearlite 
kernels or clusters of pearlite kernels in the 0.42 per cent phos- 
phorus steel specimens, and a similar tendency has been observed 
to be present in the lower phosphorus steels, though in a less striking 
manner. This phenomenon is attributed to the repellant action 
of carbon upon phosphorus when in solution in the iron. 

(c) In Figs. 13, 6 and 15, c it will be noted that the cementite 
laminae of the pearlite kernels remain uncolored while the ferrite 
lamina have been colored by etching with Stead's reagent. This 
fact proves that the cementite was little affected by the action 
of the etching medium and consequently remained uncolored or 
relatively so. It is reasonable to expect that iron phosphide or 
solid solution of phosphorus in iron would behave in a similar 
manner and consequently remain uncolored after etching with 
Stead's reagent, as has been pointed out by Stead ^^ to be the case. 

{d) Whiteley " states that a cellular structure which he was able 
to develop in his low-carbon steel specimens was due to the phos- 
phorus, this structure becoming more pronounced as the percentage 
of phosphorus is increased, and he offers the explanation of the 
formation of this cellular structure as being due to the absorption 
by gamma-iron of ferrite richer in phosphorus during the slow 
heating of the specimens up to temperatures above 815° C. 

1' stead, "Iron, carbon, and phosphorus," Jour. Iron and Steel Inst., 1915, 1, p. 140. 
16 Loc. cit. (see footnote 12), p. 377. 



Groesbeck] PhosphoTus in Open-Hearth Steels 33 

VII. SUMMARY AND CONCLUSIONS 

1. Two series of specimens, one of basic open-hearth steel and 
the other of acid open-hearth steel, with the phosphorus content 
in each series varied in four or five steps within the limits 0.008 
to 0.1 1 5 per cent, which mark the ordinary limits of phosphorus 
content in plain carbon steel, were employed in the study of the 
relationship between the phosphorus content and the microstruc- 
ture and hardness resulting from a series of different heat treat- 
ments tried. 

2. Marked irregularity in the distribution and grain size of 
both the ferrite grains and pearlite kernels was found present in 
many of the specimens, particularly in the acid open-hearth steel 
series. However, no definite relationship could be established 
between this irregularity of structure and grain size and the 
phosphorus content and also the heat treatment. 

3. No well-defined relationship could be established between 
the phosphorus content and the scleroscope hardness, though 
somewhat more satisfactory results were obtained for the Brinell 
hardness. Despite some irregularity in the results, there is a 
small but definite increase in Brinell hardness as the phosphorus 
content is raised. 

4. The irregularity in the microstructure as mentioned in para- 
graph 2 was traced to the nonuniform distribution of the phos- 
phorus in the steel. 

5. In addition to the microstructure as developed for these 
steels, a cellularlike etch pattern was formed in conjunction with 
and apparently superposed upon the ferrite and pearlite structure. 
Relationship between this cellularlike structure and the distribu- 
tion of the phosphorus content was established. 

Acknowledgment is made to Dr. Henry M. Howe, at whose 
suggestion this investigation was undertaken, for suggestions con- 
cerning the outline of the experimental procedure followed in the 
work, the use of his laboratory apparatus, and also the set of 
phosphorus steel samples which had been furnished him by Dr. 
Unger as stated in the Introduction. 

Washington, May 20, 1921. 



